It is known that the manufacture of electronic and optoelectronic devices, and more particularly those of the hightech type with materials of the III-V groups of the Periodic Table of the elements, requires the deposition of sequences of different single-crystal layers on a starting substrate, in order to obtain appropriate sequences of semiconductor materials. This deposition, prior to the actual fabrication of the individual devices or circuits, must be of the epitaxial type, namely a deposition which maintains starting crystal characteristics, and can be carried out by various techniques. These techniques, those using epitaxial growth from liquid phase (LPE), from vapor phase (VPE) or from a molecular beam (MBE) are well known.
According to this last technique, the materials to be deposited are separately evaporated in an ultrahigh vacuum environment and then sent towards the material to be grown in the shape of a more or less collimated beam. In a variant of this technique, vapors of metal-alkyls of the III group, also-called organo-metallic compounds, are used as vapour sources, ambient temperature these compounds will they present low vapour pressure, differently from the solid metals of the same group, namely Ga, In, Al. In this manner the part of the plant relevant to the vapor source can be placed outside of the ultrahigh vacuum zone. Thus ultrahigh vacuum need not be restored whenever the materials to be evaporated are to be handled. Besides in the vacuum zone there are no high- temperature furnaces for solid metal evaporation, which entail considerable complexity in the plant design to avoid possible material pollution. Finally, vapor state sources can be introduced into the vacuum zones by a single effusion source, wherein they are mixed and directed towards the substrate to be grown, instead of resorting to an effusion source with a separate furnace for each element.
The effusion sources generally used, such as those depicted at page 96 of the book entitled "Molecular Beam Epitaxy" written by M. A. Herman and H. Sitter, Springer-Verlag, edited by Morton B. Panish, are fabricated by using inlet tubes passing through a vacuum flange. The external tube side is equipped with connecting means to vapor lines, while the internal side ends in the zone for mixing the outgoing vapors. This zone is equipped with some baffle plates capable of breaking up the flows outgoing from inlet tubes to enhance vapor mixing, with an outlet nozzle to duly shape the beam directed towards the substrate to be grown and with a heater keeping a constant temperature under the control of an electric thermocouple. The necessity of maintaining the temperature controlled in the effusion source, when the surrounding environment is at the liquid nitrogen temperature, is due to the fact that vapor condensation on the walls, with consequent flow intensity alteration, is to be avoided, without reaching too high temperatures which could give rise to vapor decomposition. Temperature ranging from ambient temperature to 50.degree. C. are required.
This kind of effusion sources is rather difficult to implement, since the sources require vacuum feedthrough connectors for the heating and temperature controlling means, which actually are not justified as the temperature to be maintained inside the effusion source is rather low.